fix: handle Claude streaming error events to prevent session corruption on overload

[octo-patch]

Fixes #1463

Problem

When Claude returns an overload error (HTTP 529 or SSE error event), the session can become corrupted with an empty assistant message, causing all subsequent requests to fail:

Error: Failed to call chat-completions api

Caused by:
    messages.1: all messages must have non-empty content except for the optional final assistant message

Root cause — two issues:

1. SSE error events silently ignored: When Claude returns HTTP 200 but sends an SSE event with "type": "error" (e.g. overloaded_error), the streaming handler falls through to the _ => {} catch-all and ignores it. This results in empty output being treated as a successful completion. The empty string is then saved as an assistant message in the session, corrupting it for future requests.

2. catch_error misses {"code": N, "detail": "..."} shape: Anthropic's 529 response uses "code" as the key (not "status"), so the existing {"status": N, "detail": "..."} branch in catch_error never matched. The error fell through to the generic fallback with an uninformative message.

Solution

1. src/client/claude.rs: Add explicit handling for "error" type SSE events in claude_chat_completions_streaming. When a Claude error event arrives, bail with the error type and message instead of silently ignoring it.

2. src/client/common.rs: Extend catch_error to also match the {"code": N, "detail": "..."} response shape, producing a clear "Overloaded (status: 529)" error message.

Testing

• Built and verified the code compiles cleanly.
• The fix ensures that Claude overload errors surface as proper errors rather than silently corrupting the session with an empty assistant message.